Resist patterning techniques employed in the semiconductor lithographic process fundamental to integrated circuit manufacturing usually rely on a fluid dissolution step to remove photoresist polymer either made more soluble or left less resistant to dissolution by selective exposure to some type of photon irradiation or particle bombardment.
It is critically important to control this pattern developing dissolution carefully to achieve close dimensional control of pattern features, the tolerances of which affect yield and practicable design performance limits of semiconductor devices.
Resist develop processes attempt to hold substrate, resist, and exposing and developing system parameters fixed at optimum values in order to more predictably relate pattern dimensions to develop time employed.
In certain photoresist processes the importance of high precision temperature control of the developer and avoidance of the problems and difficulties of achieving uniform and reproducible distribution of develop solution by various spray techniques have motivated equipment and process designs employing recirculating developer solutions and provisions for complete immersion of the pattern surface of the semiconductor wafer. Such a method and design is disclosed in Pat. application Ser. No. 134,284 , filed Dec. 17, 1987, and entitled "IN-LINE SINGLE SLICE IMMERSION DEVELOPMENT PROCESS AND MACHINE".
In any such process employing recirculating developer solution, monitoring and control of the chemical reactivity of the recirculating solution is of critical importance.
Polymers employed in the formulation of photoresists typically exhibit high optical transmission at visible and nearultraviolet wavelengths and sharply lower transmission for shorter wavelengths, whereas developer fluids exhibit high transmission throughout the spectrum. At very short wavelengths, optical transmission of the developer is reduced greatly by even very small proportions of dissolved photoresist polymer. At intermediate wavelengths, greater proportions of dissolved polymer are required to cause sufficient absorption to greatly reduce optical transmission, such that sensitivity of the technique may be substantially affected by the wavelength chosen.
As photoresist developer chemical reactivity is expended by the desired chemical reaction with exposed photoresist, dissolved photoresist polymer content of the developer solution increases and may be readily and continually monitored by ultraviolet light transmission analysis. Such a method is disclosed in U.S. patent application Ser. No. 134,439, filed Dec. 17, 1987 and entitled "PROCESS AND SYSTEM FOR DETERMINING PHOTORESIST DEVELOP ENDPOINT BY EFFLUENT ANALYSIS". Thus, remaining chemical reactivity of the recirculated developer may be known by relation to polymer content, and solution dump and replenishment valves automatically controlled to maintain a preselected level of developer effectiveness.